On GTK, middle-click on the title bar (and the window frame in general) will...

chrome/browser/gtk/browser_window_gtk.cc

 // Copyright (c) 2009 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "chrome/browser/gtk/browser_window_gtk.h"
 
 #include <gdk/gdkkeysyms.h>
 #include <X11/XF86keysym.h>
 
 #include "app/resource_bundle.h"
@@ skipping 1531 matching lines @@
   // TODO(jhawkins): Investigate the possibility of the button numbers being
   // different for other mice.
   if (event->button == 8) {
     browser->browser_->GoBack(CURRENT_TAB);
     return TRUE;
   } else if (event->button == 9) {
     browser->browser_->GoForward(CURRENT_TAB);
     return TRUE;
   }
 
-  // Handle left and right clicks.  In particular, we care about clicks in the
-  // custom frame border and clicks in the titlebar.
+  // Handle left, middle and right clicks.  In particular, we care about clicks
+  // in the custom frame border and clicks in the titlebar.
   GdkWindowEdge edge;
   bool has_hit_edge = browser->GetWindowEdge(static_cast<int>(event->x),
       static_cast<int>(event->y), &edge);
   // Ignore clicks that are in/below the browser toolbar.
   GtkWidget* toolbar = browser->toolbar_->widget();
   if (!GTK_WIDGET_VISIBLE(toolbar)) {
     // If the toolbar is not showing, use the location of web contents as the
     // boundary of where to ignore clicks.
     toolbar = browser->render_area_vbox_;
   }
@@ skipping 16 matching lines @@
       if (has_hit_titlebar) {
         // Maximize/restore on double click.
         if (browser->IsMaximized()) {
           gtk_window_unmaximize(browser->window_);
         } else {
           gtk_window_maximize(browser->window_);
         }
         return TRUE;
       }
     }
+  } else if (event->button == 2) {
+    gdk_window_lower(GTK_WIDGET(browser->window_)->window);

[CalebEgg_gmail.com, 2009/08/07 09:09:38]

Won't this need to be wrapped in if (has_hit_titlebar) {} like the above and
below conditions?

+    return TRUE;
   } else if (event->button == 3) {
     if (has_hit_titlebar) {
       browser->titlebar_->ShowContextMenu();
       return TRUE;
     }
   }
 
   return FALSE;  // Continue to propagate the event.
 }
 
@@ skipping 170 matching lines @@
   // are taken from the WMs' source code.
   return (wm_name == "Blackbox" ||
           wm_name == "compiz" ||
           wm_name == "e16" ||  // Enlightenment DR16
           wm_name == "Fluxbox" ||
           wm_name == "KWin" ||
           wm_name == "Metacity" ||
           wm_name == "Openbox" ||
           wm_name == "Xfwm4");
 }